Woody Mosby, a 59-year-old semi-retired architect, got to know the concrete corridor between Dunkin' Donuts and tracks 1 & 2 of Philadelphia's Suburban train station really well this Easter weekend. Mosby was registering voters on behalf of Barack Obama, and by his count he'd gotten more than 130 forms filled out by Monday afternoon. "The deadline's today, register to vote!" Mosby, in an Oxford shirt and slacks, shouted over a flutist  a lifelong Republican Mosby had already converted  busking nearby. When people stopped he not only offered help filling out their forms, he gave them his pitch on why he changed his registration from Independent to Democrat to vote for the Illinois senator. "He's the greatest breath of fresh air I've seen in eight years, and possibly in my whole voting career," Mosby said.

Despite the hard work of dedicated volunteers like Mosby, Pennsylvania's April 22 closed primary  in which only registered Democrats may vote  is a problem for Obama, who has done better in states where Republicans and Independents can just show up at polls and register on-site. In an attempt to compensate, the Obama campaign for weeks has been running a quiet but massive effort in Pennsylvania to register tens of thousands of new Democrats, many of them traditional Independents and Republicans but also new voters. To the exclusion of virtually all else, his 150 campaign staffers and more than 2,000 volunteers across 25 offices have focused the power of Obama's grassroots organization on the registration effort, which ended at midnight Monday night when the state closed its books.

It's impossible to tell how many of the newly registered voters are Obama supporters  Hillary Clinton's campaign also made a big "March Madness Final Four Days" push to register voters, and she made surprising inroads with Republicans and independents in recent contests in Texas and Mississippi. But there is no denying that there has been a huge and sudden groundswell of registered Democrats.

Although the final numbers are not yet in, so far registration has swelled 84,801 since the 2006 elections  that's 11% of the 790,000 people who voted in the 2004 Democratic primary. Last week alone 50,347 people became Democratic voters, according to Pennsylvania's State Department, bringing the state party's total to over four million for the first time ever. Since the beginning of the year, 86,711 Republicans and Independents have switched affiliations, and in just the last three weeks 34,104 new voters registered as Democrats. Significantly, 64% of those who changed parties were in the 12 largest counties  urban areas that have large African American and educated white populations, demographics that are Obama's strength. "I think he has a chance to pull off an upset here," said Ray Owen, professor emeritus of political science at the University of Pittsburgh. "The rates of changes in registration and new registrations indicate that some independents are joining the new voters in registering Democratic."

While all of this sounds promising, Pennsylvania, with its large share of ethnic, blue-collar voters, remains an uphill battle for Obama. Clinton leads Obama by 17 percentage points, according to an average of Pennsylvania polls by the website Real Clear Politics, and polls show that most voters have already made up their minds. She has the backing of the popular Gov. Ed Rendell and the mayors of the state's two largest cities, Pittsburgh and Philadelphia, not to mention an impressive slew of congressmen, county chairmen and assemblymen, making her unquestionably Pennsylvania's establishment candidate. Clinton has also installed her A-team in the Keystone state. Mary Isenhour, a force in Pennsylvania politics, is her state director. Mark Nevins, John Kerry's state director in 2004, is also onboard, as is Nick Clemens, who ran Clinton's campaign in New Hampshire. They have 12 offices open now and expect to soon open another eight. And they have over 200 paid staff in the state.

"We're not going to take anything for granted, that's why you're seeing the investment of resources and time in this state," Kathleen Strand, Clinton's spokeswoman in Pennsylvania, told me, sitting at a folding table in Clinton's Philadelphia offices as volunteers unpacked boxes of phones, chairs and tables. "We know that Senator Obama is going to run heavy ads and is spending a lot of money in the state after, I think, initially trying to downplay the role of Pennsylvania in the process. Certainly he and his campaign are now showing that they're taking this state very seriously."

That may be true, but Obama's offices, by comparison, are a little less organized. While Clinton has had Chelsea and Bill Clinton coming in for weeks, in addition to celebrities like Ugly Betty's America Ferrera, Obama's folks have had virtually no surrogate presence in state. They trail badly in political endorsements and their volunteers spend hours blacking out the Iowa contact numbers from recycled mailers while Clinton's offices has stacks of freshly-printed glossy ones with a big photo of the candidate and Rendell on the cover.

Obama, though, is doing his best to catch up. The campaign had limited his push to radio voter registration appeals, but that changed this week when it unveiled three new tv ads: a 60-second biography spot and two 30-second issue ads; Clinton also got her first TV spot on the air Tuesday. Obama's campaign also announced Monday that he will do a six-day bus tour across the state next week. If phase one was bare-boned voter registration, phase two, get-out-the-vote, is kicking into high gear, with a focus on the relatively intimate, small group events that helped him win Iowa. "Hopefully, I'm going to be able to go back to that style of meeting people one-on-one and in small groups, having conversations," Obama said in an interview with the Philadelpia Inquirer late last week. "That approach works well for me." If he could manage to eke out a win here it would decisively knock Clinton out of the race, but the Obama campaign is quick to downplay any talk of "winning." "In our view, even if we're in a deficit in the state, if we can get it closer than it is now then we can split it in terms of the delegates," said Tommy Vietor, an Obama spokesman. "As we've always said, this is a race for delegates and we're trying to get as many as we can."

Obama chose Philadelphia to deliver his historic speech last week on race and to address concerns over his former pastor, Rev. Jeremiah Wright, after videos surfaced of Wright making controversial remarks, calling on parishioners to "God Damn America" and calling the country the "U.S. of KKKA." While the media storm has died down in the wake of Obama's speech, the effect that Wright may have on voters remains to be seen. "Is it possible Obama could win Pennsylvania? Yes, but it would take aggressive campaigning and a big shift in momentum his way," said Donald F. Kettl, a political science professor at the University of Pennsylvania. "He would need to shake off the dead weight of the Reverend Wright episode and find a way to capitalize on the economic worries that are especially strong in Pennsylvania."

Many pundits point to Obama's loss in Ohio, 45% to Clinton's 55%, as a bad omen for his chances in Pennsylvania. Both states have large white, blue-collar populations, and Ohio, at least, was an open primary. Clinton's economic-themed campaign resonates in the Rust Belt, especially under the shadow of a possible recession (though the big cities in Pennsylvania have transformed their economies from their industrial past more effectively than Ohio's). More than half  57%  of Democratic Pennsylvania voters are over the age of 45, a strong demographic for Clinton.

Still, Obama supporters are more optimistic about Pennsylvania. While Obama lost most of Ohio's rural southeast, they point out, he did win union-heavy Cuyahoga County, usually the key to Democratic victories in Ohio. "In Ohio we had about 12 days notice between the endorsement and the primary," said Anna Berger, political director of the State Employees International Union (SEIU), one of nine unions supporting Obama. "We were on the ground earlier in terms of voter registration, voter mobilization in Pennsylvania. We have 75,000 members in state and we feel we can be pretty influential in Pennsylvania." And Pennsylvania, as everyone knows, will be enormously influential in determining the next Democratic nominee for President.

Due to incorrect information provided by the Clinton campaign, the original version of this article wrongly stated that Clinton's state director in Pennsylvania is Mary Eisenhower, granddaughter of Dwight D. Eisenhower. In fact, the campaign's state director's last name is spelt Isenhour, and she is no relation to the former president. The article also had the incorrect spelling and title of Anna Burger, as well as the wrong name for the Service Employees International Union.